The present invention relates to the domain of printing heads for printers. It relates in particular to an improvement of electrostatic deflection electrodes for electrically charged ink drops. It also concerns an ink jet printer equipped with this improved head.
Ink jet printers can be divided into two major technological families, a first constituted of xe2x80x9crequest dropxe2x80x9d printers and a second constituted of continuous jet printers:
The xe2x80x9crequest dropxe2x80x9d printers are essentially office printers, intended for printing a text and graphics, in black and white or in colour.
The xe2x80x9crequest dropxe2x80x9d printers generate directly and uniquely the ink drops needed for the printing of the motives required. The printing head of these printers comprises a plurality of ink ejection nozzles, usually aligned following an alignment axis of the nozzles and each addressing a unique printing support point. When the injection nozzles are sufficient in number, the printing is obtained by simple displacement of the printing support under the head, perpendicularly to the alignment axis of the nozzles. Otherwise, a supplementary sweep of the support relative to the printer head is indispensable.
The continuous ink jet printers are usually used for industrial applications for marking and coding.
The typical function of a continuous ink jet printer can be described as follows. Electrically conducting ink maintained under pressure escapes from a calibrated nozzle, thus forming an ink jet. Under the action of a periodic stimulation device, the ink jet thus formed splits at regular time intervals at a point unique in space. This forced fragmentation of the ink jet is usually induced at a said jet break point by periodic vibrations of a piezoelectric crystal, placed in the ink and upstream of the nozzle. Starting from the break point, the continuous jet transforms into a series of identical ink drops, regularly spaced. Next to the break point a first group of electrodes is placed, called xe2x80x9ccharge electrodesxe2x80x9d, whose function is to transfer selectively, and for each drop of the series of drops, a predetermined quantity of electric charge. The group of drops of the jet then crosses a second arrangement of electrodes called xe2x80x9cdeflection electrodesxe2x80x9d forming an electric field which will modify the trajectory of the charged drops.
In a first variant, for printers called deviated continuous ink jet printers, the quantity of charge transferred to the drops of the jet is variable and each drop registers a deflection proportional to the electric charge which has previously been attributed to it. The point of the printing support reached by a drop is a function of this electric charge. The non-deflected drops are recuperated by a gutter and recycled towards an ink circuit.
Those skilled in the art also know that a specific device is required to ensure constant synchronisation between the instants when the jet is broken and the application of the charge signals of the drops. It is to be noted that this technology, thanks to its multiple levels of deflection, makes it possible for a single nozzle to print the integrality of a motive by successive segments, that is to say by lines of points of a given width. The passage from one segment to another takes place by a continuous relative displacement of the substrate compared to the printing head, perpendicular to said segments. For applications requiring a printing width slightly wider than the width of an isolated segment, several mono-nozzle printing heads, typically from 2 to 8, can be grouped together within the same housing.
A second variant of deviated continuous ink jet printers called binary continuous jet printers differ mainly from the above in that a single deflection level is created for the drops. The printing of letters or motives therefore needs the use of multi-nozzle printing heads. The centre distance between the nozzles coincides with that of the impacts on the printing support. It is to be noted that generally the drops destined for printing are the non-deflected drops. The binary continuous jet printers are intended for high speed printing applications such as addressing or personalisation of documents.
It should be emphasised that the continuous jet technique requires pressurisation of the ink, thus allowing a printing distance, that is to say the distance between the lower face of the printing head and the printing support, able to reach 20 mm, or ten to twenty times greater than the printing distances of request drop printers.
Those skilled in the art insist on optimising the performances of the layout of the deflection electrodes following two techniques.
These techniques are shown diagrammatically in FIGS. 1 to 4 in the appendix.
The first deflection technique, so-called equipotential, is the oldest. It consists of using two metallic electrodes with surfaces facing each otherxe2x80x94called active surfacesxe2x80x94. The series of drops crosses the space comprised between the active surfaces. Each of the active surfaces, relative to the jet, is raised to a constant and uniform electric potential. Two embodiments are used in particular.
The first embodiment is shown in FIG. 1.
A printer comprises a reservoir 111 containing electrically conductive ink 110 which is distributed by a distribution channel 113 towards a drop generator 116. The drop generator 116, using the ink under pressure contained in the distribution channel 113, forms an ink jet and splits this jet into a series of drops. These drops are electrically charged in a selective way by means of a charge electrode 120 fed by a voltage generator 121. The charged drops pass across a space comprised between two deviation electrodes 2, 3. According to their charge, they are more or less deviated. The drops which are least deviated or non-deviated are directed towards an ink recuperation unit or gutter 6 while the other deviated drops are directed towards a substrate 27 carried locally by a support 13. The successive drops from a burst reaching the substrate 27 can thus be deviated towards a low end position, an high end position, and successive intermediary positions. The drops of the burst as a whole form a line of width xcex94X perpendicular to an advanced position Y relative to the printing head and the substrate. The printing head is formed by the means 116 for generating and slitting the ink jet into drops, the charge electrode 120, the deviation electrodes 2, 3, and the gutter 6. This head is generally enclosed in a housing, not shown. The time between the first and second drop of a burst is very short. The result is that despite continuous movement between the printing head and the substrate, it can be considered that the substrate has not moved relative to the printing head during the time of a burst. The bursts are fired at regularly spaced intervals. The combination of the relative movement of the head and the substrate, and the selection of the drops of each burst directed towards the substrate make it possible to print any motive such as that shown as 28 in FIG. 1. In the following description only the deviation electrodes of the drops of series 1 of drops formed from an ink jet exiting from the nozzle will be considered.
Concerning the deviation of said drops, it is a matter of forming a very strong electric field Ed, by application of a voltage Vd, which is constant between the two electrodes 2, 3 formed by two parallel plates 2, 3. The value of the electric field Ed created between the active surfaces of the electrodes 2, 3 is called optimal when this value is slightly lower, by subtracting a security margin, compared to that of the breakdown field corresponding to the space e between the active surfaces.
Such a concept is characterised by its simplicity but also by numerous inconveniences:
a high value of e, typically 5 mm, is indispensable for allowing the printing of very wide segments at the usual printing distances. Such a spacing implies the use of a very high value of Vd, about 8 kV, which cannot be generated within the printing head because of lack of space, requires complicated connectics and generally leads to raising each of the electrodes to potentials of opposite signs relative to the reference potential of the ink;
such a value for the potential difference also makes it necessary to respect the minimum spacing from other metallic elements of the printing head, for example charge electrodes, recuperation gutter or housing, in order to avoid any electrical breakdown. The overall size resulting leads to the path of the drops being lengthened needlessly, and thus the time during which aerodynamic or electrostatic perturbations can act, which is detrimental to the precision of the impacts on the printing support;
it is known to those skilled in the art that the value of the breakdown field between two electrodes plunged in a gaseous medium such as air, is a decreasing function of the spacing e between the two electrodes. The high value of e characterising this first embodiment and the restrictions relative to avoiding breakdown limit the value of the deflection field Ed to a value lower than the optimum value. The printing of wide segments thus requires high deflection plates, typically 25 mm, so as to obtain the maximum deflection required by the longer action of the electric field. This characteristic also contributes to lengthening the path of the drops towards the printing support.
The second deflection technique, shown diagrammatically in FIG. 2, is differentiated from the above by the fact that at least one part of at least one of the two active surfaces forms a non-zero angle with the ink jet axis 1. The geometry is among that most usually encountered and is very simple. In a part 15 upstream of a layout 20 of two electrodes 2, 3 formed by plates 2, 3, the plates are parallel and spaced by a distance generally less than that adopted in the first embodiment. The electric field in this upstream part, 15, between the two plates 2, 3, then reaches a level at least equal to that of the first mode but for a lower potential difference. Then it becomes necessary, in order to allow printing of wide segments, to avoid the most charged drops, and thus the most deviated, to enter into collision with the electrode 3 towards which they are deviated. The solution retained consists of inclining, relative to the axis of the jet, a downstream part 16 of this electrode 3. It is evident that in the downstream region, the value of the electric field drops very significantly, is no longer optimum, which results in significant lowering of the deflection efficiency. Consequently, the main advantage of the second variant in comparison with the first is to provide almost equivalent performances for a lower potential difference.
One can refer to patent applications WO 89/03768 and WO 98/28148 in order to obtain supplementary details about the incorporation of such deflection devices within binary or deviated continuous ink jet printers. In this latter technology, it can be noted that one of the two deflection electrodes is often suppressed.
The patent application FR 77 33131 proposes a variant, shown in FIG. 3, in which the active surface, towards which the deflection of the drops is oriented, has a double longitudinal and transversal curvature. The convexity resulting from the adoption of these curvatures makes it possible to eliminate any metallic sharp edges and thus to minimise the risks of electric breakdown. The longitudinal curvature of the active face 17 of the electrode 3 also provides improved transition between the upstream region 15 with strong electric field and the downstream region 16 with low electric field.
In order to maintain optimum deflection efficiency all along the path of the drops, a second technical path, so-called xe2x80x9cnon-equipotentialxe2x80x9d has been thought of, in which at least one of the two active surfaces 2, 3 is raised to a constant but non-uniform electric potential. The patent application GB 2 249 995 A shows two different concepts following this idea. The first, in the diagram of FIG. 4, operates two plane metallic electrodes 2, 3 between which a potential difference Vd is created. On one, 3, of these electrodes 2, 3 a part 18 made out of a dielectric substance is added, whose shape is similar to that of a portion of an elliptic cylinder. A curved face 19 of this part is placed facing the jet 1 and constitutes the active surface of the deflection device on which the electric potential is not uniform. The permittivity of the dielectric substance being knownxe2x80x94and greater than that of airxe2x80x94it is suggested in the document to adjust the curve of the part 18 in such a way as to follow simultaneously the trajectory of the most highly charged drops and to obtain an optimum value of Ed at any point comprised between the two active surfaces of the device.
The implementation of this device brings up problems:
cost the supplementary part 18 of complex shape and with a very good surface appearance is necessary;
manufacturing: as well as respecting the dimensional tolerances, the transfer of the dielectric part 18 requires gluing resistant to ink sprays.
operation: the active surface 19 of the dielectric part 18 does not permit the evacuation of parasitic electric charges coming from the ambient gaseous medium or the ink droplets accidentally projected on the wall. The accumulation of these electric charges rapidly leads to strong degradation of the field strength Ed.
A variant, proposed in the U.S. Pat. No. 4,845,512 A, consists of replacing the dielectric substance by an electret in order to be independent from the voltage generator creating the potential difference Vd. This concept remains subject to the same criticisms as the others mentioned above.
The second concept presented in the patent GB 2 249 995 A suggests the use of a resistive substance for forming the active face of one of the two electrodes of the deflection device. It is suggested that one should obtain, through careful alimentation of this electrode at its two extremities, a variation of electric potential along its active surface. This non-uniformity should then generate a deflection field Ed such that its value would be approximately optimum at each of the points comprised between the two active surfaces of the device. This solution is criticised in said patent GB 2 249 995 A by emphasising the high current consumptionxe2x80x94and therefore the high heat emissionxe2x80x94which its implementation would induce.
Patent FR 97 06799 includes an analysis and detailed appraisal of the above proposals. This document insists essentially on describing a non-equipotential device exempt from the operational difficulties described above. To this effect, at least one of the two active surface is made under the form of an insulating substrate on which is deposited, according to the height of this surface, a plurality of electrodes connected to different voltage sources. A resistive coating covers the insulating substrate and the electrodes. Careful choice of the number of electrodes, of the value of the voltages applied and of the value of the sheet resistance of the resistive coating makes it possible to create an optimum field Ed over the whole height of the deflection device while still minimising and controlling the electric currents and the parasitic heat fluxes.
The major handicap of such a device resides in its complexity of production and its manufacturing cost.
To resume, the deflection devices representative of prior art and implemented in ink jet printers are characterised as follows:
equipotential way: simple concept but poor deflection efficiency.
non-equipotential way: increased deflection efficiency but implementation difficult because of the manufacturing costs and the operational principles adopted.
Compared to the state of the art described above, the aim of the present invention relates to producing an electrostatic deflection device that can be integrated into a printing head of an ink jet printer, and whose efficiency equals or exceeds that of non-equipotential designs for significantly lower production costs, by means of an arrangement of deflection electrodes whose active surfaces are raised to uniform electric potentials.
Another aim of the present invention is to constitute an arrangement of deflection electrodes with reduced overall dimensions and leading to a reduction of the overall dimensions of a printing head of a printer in which this head is incorporated.
Another aim of the present invention is to obtain deflection performances with a voltage that is significantly lower than the usual voltages feeding equipotential deflection electrodes and thus facilitating integration of said electrodes and a generator of said lower voltage in a printing head.
A further aim of the invention is to reduce significantly the risk of accidental projection of ink on the active surface of the deflection electrodes.
For all these purposes, the invention relates to a printing head for a continuous ink jet printer equipped with means for generating an ink jet according to an axis of the ink jet, from at least one ejection nozzle of the jet, and for splitting the jet into a series of drops, means for charging the different drops in the series of drops in a selective way, and deviation electrodes for charged drops, deviating the drops in function of the value of the charge received, either towards a gutter for recuperating the drops, or to an impression substrate maintained locally by an impression substrate support, the deviation electrodes each having an upstream part and a downstream part relative to the ejection nozzle of the jet, an active surface of each deviation electrode being a surface of said electrode facing the series of drops, the printing head characterised in that the deviation electrodes of the drops of the jet comprise two electrodes, a first and a second, the active surface of the first electrode having a first concave longitudinal curvature whose local radius of longitudinal curvature is located in a plane formed by the axis of the ink jet and a deviation direction of the drops, in that the active surface of the second electrode has a first convex longitudinal curvature and in that the first electrode has a recess with a border in its downstream part.
The meaning of downstream part will now be explained. The function of the recess is to allow the passage of non-deviated or slightly deviated drops through the first electrode. The non-deviated drops then closely follow a trajectory which, as a first approximation, can be considered as rectilinear. The result of this is that the most upstream part of the recess border will be located immediately next to and slightly upstream of the point of intersection of the first electrode with the axis of the jet. The most upstream part of the border of the recess must therefore be located at a sufficient distance from the point of intersection of the first electrode with the jet axis so that a non-deviated drop can pass through the recess in the electrode with a quasi-zero probability of intercepting the electrode.
The slightly charged and thus slightly deviated drops have a trajectory whose curvature can be lower than that of the first electrode. The trajectory of the slightly deviated drops is therefore likely to be secant at the active surface of the first electrode. The recess must be such that it allows the passage of these slightly deviated drops. The possible point of intersection of the trajectory of a little-deviated drop and the surface of the electrode before the recess is necessarily located downstream of the point defined above as being the most upstream point of the recess. It can thus be considered that the downstream part of the first electrode is a part of this electrode located downstream of the point of intersection of the electrode and the axis of the jets.
Given the function of the recess, it can also be understood that the shape of this recess will be such that its line of symmetry is a line defined by the intersection of the electrode before the recess, with a plane containing the axis of the jets and the direction of deviation of the drops. The recess will thus have an oblong shape centred on the line of symmetry defined above.
The width of the recess results from a compromise between two demands, letting the drops pass through the first electrode without risk of collision between the drop and the electrode, which means that the recess should be wide, and not reducing too much the inter-electrode field, which means that the recess should be narrow.
The diameter of the drops of ink is of the order of several tens of xcexcm, typically comprised between 30 and 140 xcexcm, for example 100 xcexcm.
The width measured perpendicular to this line is greater than the diameter of the drops and ideally of the order of two to three times the diameter of the drops, that is typically 200 to 300 xcexcm. However, to be certain of avoiding collisions between drops and the first electrode, one may have to set a width of the order of 8 to 10 times the diameter of the drops.
Thus the embodiments of the invention can, together or separately, present the following characteristics.
The curvature of the second electrode is such that the active surface of the second electrode is substantially parallel to that of the first electrode so that the two active surfaces have a closely constant spacing e between them.
The border of the recess has a highest upstream point located in the neighbourhood of the intersection, before the recess, of the first electrode with the axis of the ink jet.
The recess has a symmetry relative to a plane containing the axis of the ink jet.
The recess has a width comprised between two and ten times the diameter of the ink drops.
The recess has the shape of an oblong slit with one opening extending onto the most downstream part of the first electrode.
The spacing between the active surfaces of the two electrodes is substantially constant from the upstream to the downstream of the electrodes and comprised between 4 and 20 times the diameter of the ink drops, that is between about 0,5 and 3 mm.
A most downstream edge of the first electrode is closer to the printing support than a most downstream surface of the recuperation gutter.
The second electrode is provided, from its active surface, with a groove traced according to an axis contained in a plane containing the axis of the jet.
A base of the groove is connected to the active surface of the second electrode by a surface curved transversally according to curvature radii of a value greater than the radius of the ink drops.
The tongues of the first electrode formed on either side of the recess and the second electrode are curved transversally according to the curvature radii with a value greater than the radius of the ink drops.